Bluetooth: Classic: Fix LTK cannot be derived issue

The LTK cannot be derived by LK when LK is not weaker than the old LTK.

Improve the function `smp_br_pairing_allowed()` to avoid the LTK be
overwrote when old LTK has MITM protection but new LK has not MITM
protection.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>